Illuminator apparatus

ABSTRACT

To provide a reflection type liquid-crystal display device having an illuminator in front of the liquid-crystal display device, the illuminator having a light guide plate superior in light utilization efficiency, dots constituted by a plurality of small recess or protrusion portions for reflecting the light incident on the light guide plate toward the liquid-crystal display device are formed in or on a surface of the light guide plate which does not face the liquid-crystal display device. Each of the dots is set to have a substantially V-shape in section, the sectional inclination angle of the V-shape is set in a range of from 35 to 43°, and the sectional vertex angle of the V-shape is in a range of 70.6±2.5°.

BACKGROUND OF THE INVENTION

The present invention relates to an illuminator provided with a lightguide plate, and to a liquid-crystal display device using theilluminator.

Recently, portable electronic apparatuses represented by a portableinformation terminal, a portable telephone, etc. have been made smallerin size and lower in price. As a result, such portable electronicapparatuses have come into wide use.

Specific examples of such a portable electronic apparatus include areflection type liquid-crystal display device which is effective inreducing power consumption, and a liquid-crystal display device using areflection type liquid-crystal display element and a front-lightingilluminator.

As the performance required of this illuminator, the irradiationquantity of the light which irradiates a liquid-crystal panel has to belarge and the whole surface of the liquid-crystal panel has to beirradiated uniformly. The enhancement of the irradiation quantity oflight is achieved easily by the increase of the quantity of the lightradiated from a light source. However, such a method cannot be regardedas practical because it is accompanied by the increase of powerconsumption.

As the background art concerning such an illuminator, JP-A-10-188636discloses a method in which a light source is disposed in an end portionof a light guide plate made of a transmissive material and smallprotrusion portions for taking the light out toward a liquid-crystaldisplay device are formed on the lower surface of the light guide plate,as shown in FIG. 1.

In addition, JP-A-11-53918 discloses another method in which smallprotrusion portions (or small recess portions) for reflecting the lightentering a light guide plate toward a liquid-crystal display device areformed on the upper surface of the light guide plate, as shown in FIG.1.

Further, JP-A-11-72787 discloses a further method in which smallprotrusion portions or small recess portions for transmitting the lightentering a light guide plate toward a liquid-crystal display device areformed at random on the lower surface of the light guide plate.

SUMMARY OF THE INVENTION

The following properties are required of an illuminator for use in thecondition that it is disposed in front of a liquid-crystal displaydevice.

(1) Haze (turbidity or cloudiness) of a light guide plate is low.

(2) The surface reflectivity is low.

(3) The light entering eyes directly from the light guide plate used inthe illuminator is less.

(4) The exiting angle of the light made to go out from the lower surfaceof the light guide plate used in the illuminator is small.

(5) No moire pattern is produced.

In the above-mentioned background art disclosed in JP-A-10-188636,however, the exiting angle of the light made to go out from the lowersurface of the light guide plate becomes large due to the sectionalshape of each of the protrusion portions. Therefore, there is such aproblem that not only is the light entering the liquid-crystal displaydevice less, but also the protrusion portions are arranged regularly sothat moire is produced easily.

On the other hand, in the background art disclosed in JP-A-11-53918, theaforementioned problem can be improved somewhat, but the light enteringeyes directly from the light guide plate is apt to be generated due tothe shape of each of the protrusion portions (or recess portions). As aresult, a problem in visibility as a liquid-crystal display device stillremains. In addition, moire is produced easily due to the arrangement ofthe protrusion portions.

Further, in the background art disclosed in JP-A-11-72787, the exitingangle of the light made to go out from the lower surface of the lightguide plate becomes large due to the sectional shape of each of theprotrusion portions. Accordingly, not only is the light entering theliquid-crystal display device less, but also it is difficult to disposea large number of small protrusion portions or small recess portionsirregularly.

The present invention has been developed to solve the foregoingproblems. It is an object of the present invention to provide anilluminator which can enhance the irradiation quantity of the lightwhich irradiates a liquid-crystal display device without increasing thequantity of the light radiated from a light source; a method formanufacturing the illuminator; and a liquid-crystal display device usingthe illuminator.

In order to attain the foregoing object, an illuminator disposed infront of a liquid-crystal cell according to the present invention isconstituted by a light guide plate and a light source disposed on one ofside surfaces of the light guide plate. This light guide plate includesan incidence surface on which the light from the light source isincident, and a light transmission surface through which the incidentlight on the light guide plate is made to exit to the liquid-crystalcell. In addition, a plurality of dots each constituted by a smallrecess portion or a small protrusion portion for reflecting the lightincident on the incidence surface toward the light transmission surfaceare formed on the surface opposite to the light transmission surface.Each of the dots has a substantially V-shape in section, and aninclination angle of the section is in a range of from 35 to 43°. Avertex angle of each of the dots is set to be in a range of 70.6±2.5°.

In addition, according to the present invention, each of the dots issubstantially rectangular in plan shape, and each of the dots is set tohave a short-side length in a range of from 0.002 to 0.05 mm and along-side length in a range of from 0.002 to 0.2 mm.

Then, the dots each having such a shape are disposed at random on thesurface opposite to the light transmission surface of the light guideplate.

Further, according to the present invention, the light guide plateconstituting the illuminator includes an incidence surface on which thelight from the light source is incident, and a light transmissionsurface through which the incident light on the light guide plate ismade to exit to the liquid-crystal cell, and a plurality of dots eachconstituted by a small recess portion or a small protrusion portion forreflecting the light incident on the incidence surface toward the lighttransmission surface are formed on the surface opposite to the lighttransmission surface.

Then, not smaller than 95% of the whole area of the surface on which thedots are formed is sectioned into 0.25 to 1 mm² square areas, and thedots are disposed in each of the square areas so that a function G(R)which is obtained by taking a weighted average of a radial distributionfunction g(R) obtained for each of the dots in accordance with anarrangement relationship of the dots, and which is obtained byapproximating the weighted average by a least squares method satisfies arelation of 0<S₁/S₂<0.2 in a range of R/R₀=3 to 6.

Provided that R designates a distance from a central position of one dotto a central position of another dot; R₀, a value obtained by dividing alength of one side of the square area by a square root of the number ofthe dots existing in the square area; S₁, a value obtained byintegrating a difference between G(R) and an average value of G(R) withR/R₀ which is in a range of from 3 to 6; and S₂, a value obtained byintegrating the average value of G(R) with R/R₀ which is in a range offrom 3 to 6.

In addition, each of the dots is disposed so that the function G(R) issubstantially 0 in a range of R<(short-side length of dot)×2, at leasttwo peaks exist in the function G(R), and two peaks each of which is atleast twice as large as the average value of the function G(R) exist ina range of R/R₀=3 to 6.

Moreover, according to the present invention, an oxide film is formed ona surface of a silicon substrate having a predetermined crystal planeand a resist film is formed on the oxide film so that each dot to beformed on the light guide plate has a V-shape in section and aninclination angle of the section is in a range of from 35 to 43°. Then,a dot pattern is formed on the oxide film with the resist film servingas a mask. Then, after anisotropic etching is given to the siliconsubstrate with the oxide film serving as a mask, a metal film is furtherformed on the silicon substrate. Further, the metal film is stripped offso as to produce a stamper or a replica thereof. Dots are transferredonto a surface of a film or plastic by use of the stamper or thereplica. Thus, a light guide plate having such dots is formed.

A liquid-crystal display device according to the present invention hasan illuminator which has such a light guide plate, a liquid-crystaldisplay portion and a control portion. The illuminator is disposed infront of the liquid-crystal display portion so that external light istransmitted through the illuminator and enters the liquid-crystaldisplay portion. The quantity of light with which the illuminatorirradiates the liquid-crystal display portion is controlled by thecontrol portion in accordance with the quantity of the external light.

Further, a portable electronic apparatus using the liquid-crystaldisplay device according to the present invention has a light receivingportion. The illuminator is controlled by use of the quantity of theexternal light received by the light receiving portion so that theluminance of the liquid-crystal display portion is made substantiallyconstant.

Furthermore, the portable electronic apparatus also has a signalreceiving portion. The illuminator is controlled by the control portionwith a signal supplied to the signal receiving portion as a trigger, sothat the liquid-crystal display portion is irradiated with light inaccordance with the external light entering the light receiving portion.Thus, the luminance of the liquid-crystal display portion is madesubstantially constant.

BRIEF DESCRIPTION OF THE DRAWINGS

These and other features, objects and advantages of the presentinvention will become more apparent from the following description whentaken in conjunction with the accompanying drawings wherein:

FIG. 1 is a perspective view of an illuminator for explaining a firstembodiment;

FIG. 2 is a sectional view of the illuminator for explaining the firstembodiment;

FIG. 3 is a sectional view and a plan view of a micro-dot (small recessportion);

FIG. 4 is a sectional view and a plan view of a micro-dot (smallprotrusion portion);

FIG. 5 is a view for explaining tracks of light propagated inside alight guide plate according to the first embodiment;

FIG. 6 is a conceptual view for explaining an arrangement of micro-dots;

FIGS. 7A and 7B are views for explaining sectional inclination anglesand sectional vertex angles of micro-dots respectively;

FIG. 8A is a graph showing a relationship between the sectionalinclination angle of the micro-dots and efficiency of the light made toexit from the light guide plate;

FIG. 8B is a graph showing a relationship between the sectionalinclination angle of the micro-dots and the exiting angle of the lightmade to exit from the light guide plate when the angle of the lightpropagated inside the light guide plate is in a range of ±25°;

FIG. 8C is a graph showing a relationship between the sectionalinclination angle of the micro-dots and the exiting angle of the lightmade to exit from the light guide plate when the angle of the lightpropagated inside the light guide plate is in a range of ±35°;

FIGS. 9A and 9B are views for explaining the influence of the range ofthe exiting angle of the light made to exit from the light guide plate,respectively;

FIGS. 10A to 10C are views for explaining sectional shapes of micro-dotsrespectively;

FIGS. 11A to 11D are views for explaining other forms of the micro-dotrespectively;

FIGS. 12A to 12C are views for explaining a method for arranging themicro-dots;

FIG. 13 is a view for explaining a problem in the arrangement ofmicro-dots;

FIG. 14 is a graph for explaining a distribution of a radialdistribution function G(R);

FIGS. 15A to 15G are views showing a flow of steps of a method formanufacturing a light guide plate having micro-dots, up to the step offorming a dot pattern;

FIGS. 15H to 15M are views showing the flow of steps of the method formanufacturing a light guide plate having micro-dots, up to the step ofproducing the light guide plate by use of an etching injection-moldingmethod;

FIGS. 16A and 16B are views for explaining the production of a siliconsubstrate having a predetermined crystal plane, respectively;

FIG. 17 is an explanatory view of the silicon substrate having thepredetermined crystal plane;

FIG. 18 is a view for explaining a liquid-crystal display device with anilluminator disposed in front of the display device according to asecond embodiment;

FIG. 19 is a schematically sectional view of the liquid-crystal displaydevice with the illuminator disposed in front of the display device;

FIG. 20 is a schematically sectional view of a portable electronicapparatus according to a third embodiment; and

FIG. 21 is a graph for explaining the relationship between the frontalluminance of the liquid-crystal display device and the quantity ofexternal light.

DETAILED DESCRIPTION OF EMBODIMENTS

Embodiments of the present invention will be described below in detailwith reference to the drawings.

FIG. 1 is a perspective view of an illuminator for use in aliquid-crystal display device according to a first embodiment. FIG. 1also includes a perspective view of a plurality of small recess portionsor small protrusion portions (hereinafter referred to as “micro-dots”)surrounded by a light guide plate flat portion, for changing thetravelling direction of light in the light guide plate. Incidentally,FIG. 1 illustrates the case where the dots are formed into small recessportions.

A light guide plate 1 is disposed in front of a liquid-crystal displaydevice 8 so that the light from light sources 6 disposed on one of sidesurfaces of the light guide plate 1 is made incident on the light guideplate 1 and this incident light is reflected toward the liquid-crystaldisplay device 8 by dots 5 provided on an upper surface 7 of the lightguide plate 1.

FIG. 2 is a sectional view conceptually showing the relationship betweenthe illuminator and the liquid-crystal display device. As describedabove, the light from the light sources 6 reflected by the dots 5 (thelight 3 made to exit from the lower surface of the light guide plate)enters the liquid-crystal display device 8, and is reflected by areflection plate provided in the liquid-crystal display device 8. Then,the reflected light is transmitted through the light guide plate 1 againand reaches eyes of an observer.

Incidentally, of the light entering the light guide plate IL from thelight sources 6, the light 2 shown in FIG. 2 reaches the eyes of theobserver directly without travelling through the liquid-crystal displaydevice 8. In consideration of the object of the illuminator described inthis embodiment, not to say, the light 2 reaching the eyes of theobserver directly is not preferable, and it should be reduced to theutmost.

Detailed description will be made about the shape of the micro-dots 5for attaining the foregoing object.

FIG. 3 shows a sectional view and a plan view of a micro-dot 5 which isa small recess portion by way of example. On the other hand, FIG. 4 is asectional view and a plan view of a micro-dot 5 which is a smallprotrusion portion. In addition, FIG. 5 is a view for explaining thetracks of light propagated in the light guide plate in this embodiment.

Description will be made about the case where each of the micro-dots 5is a small recess portion by way of example. The illuminator shown inFIGS. 3 and 5 is provided with light sources 6 and a light guide plate1. In addition, as described above, the illuminator is disposed in frontof a liquid-crystal display device 8, and the light sources 6 aredisposed on one of side surfaces of the light guide plate 1. Themicro-dots 5 each shaped as shown in FIG. 3 are formed on an uppersurface 7 of the light guide plate 1. That is, as shown in FIG. 5, themicro-dots 5 are formed on the surface (the light-guide-plate uppersurface 7) of the light guide plate 1 which does not face theliquid-crystal display device 8.

This dot arrangement is to use the micro-dots 5 to reflect the light 9incident on the side surface of the light guide plate 1 toward theliquid-crystal display device 8. Specifically, in FIG. 6, the lightexiting from the light source 6 enters, as the light-guide-plateincident light 9, the light guide plate 1 through an incidence endsurface of the light guide plate 1 so as to form light-guide-plateguided light. The light-guide-plate guided light travels toward theother end surface of the light guide plate 1 while total reflection isrepeated between a light-guide-plate lower surface 10 and thelight-guide-plate upper surface 7.

Of the light-guide-plate guided light, the light 18 travelling to areflection slope 11 of the corresponding micro-dot 5 is reflected by theslope so as to travel toward the light-guide-plate lower surface 10. Thelight reaching the light-guide-plate lower surface 10 exits from thelight guide plate 1 while being refracted by the light-guide-plate lowersurface 10. The refracted light enters the liquid-crystal display device8 as illumination light.

Thus, by use of the light guide plate 1 described in this embodiment,the light from the light source 6 can be made to exit toward theliquid-crystal display device 8 efficiently.

At this time, when a sectional slope angle 12 of the correspondingmicro-dot 5 is selected so that the light reflected by the micro-dotreflection slope 11 satisfies the condition of total reflection, thelight 19 entering the eyes of the observer directly from the light guideplate 1 can be reduced. As a result, the quantity of the light madeincident from the light guide plate 1 to the liquid-crystal displaydevice 8 becomes so large that a reflection type liquid-crystal displaydevice with good visibility can be realized.

The intensity of the light made to exit from the light source 6 isgenerally reduced in the light guide plate 1 as the distance from thelight source 6 increases. Accordingly, the intensity of the lightentering the liquid-crystal display device 8 can be uniformalized, forexample, by changing the density of the micro-dots 5, that is, bychanging the number of the micro-dots 5 per unit area, or by changingthe size or length of the micro-dots 5 while keeping the density of themicro-dots 5 uniform, or by a combination of the aforementioned methods.

In the case of a single light source, it is preferable that the densityof the micro-dots 5 is formed to increase, in accordance with theexponential function or power law, from the light-source-side endsurface of the light guide plate toward the opposite end surface of thelight guide plate.

The sectional shape of each of the micro-dots 5 is substantiallyperpendicular to the upper surface 7 of the light guide plate on whichthe micro-dots 5 are formed, as shown in FIG. 6. The sectional shapetaken on a plane substantially in parallel with the travelling directionof the light entering the light guide plate 1 from the light source 6(the travelling direction of the guided light in the light guide plate)is substantially a V-shape. The sectional inclination angle 12 is set tobe in a range of from 35 to 43°, and the sectional vertex angle is setto be in a range of 70.6±2.5°. Incidentally, in the case of point lightsources, it may be considered that there is a linear light sourceconnecting those point light sources.

Here, the micro-dot reflection slope 11, the sectional inclination angle12, dot depth (dot height) H, dot short-side length W15 and dotlong-side length L16 are defined in FIGS. 3 to 4 and FIGS. 7A and 8B.That is, as shown in FIGS. 7A and 7B, the sectional inclination angle 12is set to be an angle between the light-guide-plate upper surface 7 anda straight line connecting points a and b on the micro-dot reflectionslope 11 when the micro-dot depth (height) H14 is divided to three equalparts. In addition, the vertex angle 17 is set to be an angle betweenthe straight line connecting the points a and b and a straight lineconnecting points C and D on the micro-dot reflection slope 11.

The reason why the sectional inclination angle 12 is set to be in arange of from 35 to 43° is to reduce the light 2 (see FIG. 2) enteringeyes directly from the light guide plate 1 and to increase the quantityof the light entering the liquid-crystal display device 8 so as toimprove the visibility of the liquid-crystal display device 8.

That is, as shown in FIG. 5, light with a divergent angle within about±35°, more particularly within about ±25° with respect to the horizontaldirection in which the light is travelling is generally propagatedinside the light guide plate 1. The light incident on the reflectionslope 11 of the micro-dot 5 is reflected and refracted by the slope 11.Of the reflected and refracted light, the reflected light changes itstravelling direction downward and exits from the light-guide-plate lowersurface 10 so as to function as illumination light for theliquid-crystal display device 8.

On the other hand, the light refracted and transmitted by the reflectionslope 11 of the micro-dot 5 exits from the upper surface 7 of the lightguide plate 1 without entering the liquid-crystal display device 8.Thus, the light enters the eyes of the observer. Therefore, the observerdirectly views the light from the light sources 6 so as to form a brightspot or a bright line, which lowers the value as a liquid-crystaldisplay device. Further, since the aforementioned light does notilluminate the liquid-crystal display device 8, the utilizationefficiency of the light from the light sources 6 is reduced.

It is therefore necessary to set the sectional inclination angle 12 sothat reflection by the light-source-side slope of the micro-dot 5satisfies the total reflection condition to the utmost. As a result, itis possible to utilize the light from the light sources 6 largely forilluminating the liquid-crystal display device.

FIG. 8A shows the relationship between the sectional inclination angleand the efficiency when the light from the light sources is made to exitfrom the light-guide-plate lower surface. On the other hand, FIGS. 8Band 8C show the relationship between the sectional inclination angle andthe range (half width) of the exiting angle of the light made to exitfrom the light-guide-plate lower surface. Incidentally, in each case,the propagation angle with which light travels while reflected by theupper and lower surfaces of the light guide plate was varied within±35°, and more particularly within ±25°.

As is apparent from the result of FIG. 8A, when the propagation angle oflight is in a range between ±35° and the sectional inclination angle isin a range of from 28 to 43°, the exiting efficiency of the light fromthe light guide plate can be made large.

However, if the sectional inclination angle is not larger than 35°, theexiting angle of the light made to exit from the light-guide-plate lowersurface becomes large. This large exiting angle is the main cause oflowering of the contrast of the liquid-crystal display device and oflowering of the frontal luminance of the liquid-crystal display device.That is, if the sectional inclination angle is about 30°, the exitingangle of the light 3 made to exit from the light-guide-plate lowersurface is in a range of from 18 to 38° (see FIGS. 8B and 8C), the light50 reflected by the light-guide-plate-1-side surface of theliquid-crystal display device 8 is apt to be produced as shown in FIG.9A, so as to cause the lowering of the contrast. Further, the exitingangle of the display light 51 from the liquid-crystal display devicewith respect to the liquid-crystal display device is apt to increase.Thus, the frontal luminance is lowered.

On the contrary, if the sectional inclination angle is about 40°, asshown in FIG. 9B, the exiting angle of the light 3 made to exit from thelight-guide-plate lower surface is in a range of from 5 to 9° (see FIG.8B) or in a range of from 4 to 12° (see FIG. 8C). As a result, the light3 is difficult to be reflected by the light-guide-plate-1-side surfaceof the liquid-crystal display device 8 so that the contrast is enhanced.Further, since the exiting angle of the display light 51 from theliquid-crystal display device with respect to the liquid-crystal displaydevice is also reduced, the frontal luminance is enhanced.

When the sectional inclination angle is set to be not smaller than 43°,the light exiting angle becomes small, but the light exiting efficiencyis reduced as shown in FIG. 8A. This is because the total reflectioncondition is not satisfied at the time of reflection by the micro-dotreflection slope 11. Accordingly, the light 2 entering the eyes of theobserver directly from the light guide plate as shown in FIG. 2increases undesirably to be the main cause of lowering of thevisibility.

From the above points, a preferable angle as the sectional inclinationangle is in a range of from 35 to 43° in which the light exiting angleis small, the range of the exiting angle is narrow and there is noreduction in the exiting efficiency.

Although the sectional inclination angle was set to be in a range offrom 35 to 43° in this embodiment, it is more preferable that thesectional inclination angle is set to be in a range of 39 to 42° inwhich the range of the light exiting angle becomes minimal and opticaldesign becomes easy.

Incidentally, the propagation angle of the light propagated inside thelight guide plate 1 is generally about 35° when a cathode-ray tube and areflector are used as the light source 6. Alternatively, if a lightemission diode is used as the light source 6, the propagation angle isapproximately in a range between ±35° though it depends on the lensdesign of the light emission diode, and so on. It is therefore importantto determine a suitable sectional inclination angle in accordance withthe kind of the light source in consideration of the results of FIGS. 8Ato 8C and FIGS. 9A and 9B.

On the other hand, the reason why each of the micro-dots 5 is formedinto a substantially V-shape in section is to reduce the haze (turbidityor cloudiness) of the light guide plate 1. Then, as shown in FIGS. 10Ato 10C, respectively, the substantially V-shape includes a V-shape, aV-shape rounded at the vertex and a substantially U-shape, which areformed so that an R portion 20 of the micro-dot 5 is not larger thanabout 20% of the dot depth (height) H14.

With such a sectional shape, slopes which do not contribute toreflection are reduced to the utmost (the area viewed from thelight-guide-plate upper surface 7 is small) when the light entering thelight guide plate is reflected by the reflection slopes.

It is preferable that the vertex angle of the sectional shape of eachmicro-dot 5 is set to be in a range of 70.6±2.5°. This is because a moldfor forming micro-dots in the light guide plate can be produced easilyby use of anisotropic etching technique of silicon single crystal aswill be described later.

In addition, it is preferable that the plan shape of each micro-dot isformed to be substantially rectangular. Here, the substantiallyrectangular shape includes a rectangle and a rectangle rounded at itscorners. Alternatively, the shape may be a square. Particularly, whenthe shapes of the micro-dots 5 are substantially rectangular, scatteredlight inside the light guide plate 1 is reduced so that the lightexiting efficiency or the like is enhanced. At the same time, the areaof slopes which do not contribute to reflection in the micro-dots 5 isreduced in comparison with that in the case of a circular shape or thelike. As a result, there is an effect of reducing the haze of the lightguide plate.

Incidentally, although description was made in the aforementionedembodiment about the case where the micro-dots 5 were small recessportions, effects similar to those in the aforementioned case can beobtained even if each micro-dot has a protrusion portion as shown inFIG. 4, or a square pyramidal or wedge shape as illustrated in FIGS. 11Ato 11D.

Next, description will be made about the arrangement of the micro-dots.

The dot arrangement is preferably made so that the long-side directionof each dot 5 is substantially parallel with the longitudinal directionof a cathode-ray tube or the like serving as the light source 6 as shownin FIG. 12A. Alternatively, if a single point light source such as alight emission diode is used as the light source 6, it is preferablethat the long-side direction of each dot 5 is made substantiallyparallel with a tangential direction of a circle around the point lightsource as shown in FIG. 12B. If a plurality of point light sources suchas light emission diodes are used as the light sources 6, the long-sidledirection of each dot 5 is made substantially parallel with a straightline connecting the plurality of point light sources 6 as shown in FIG.12C.

The reason is as follows. Most of the light beams entering the lightguide plate 1 from the light sources 6 travel substantiallyperpendicularly to the longitudinal direction of the light sources 6.After the light beams are incident on the slopes 11 of the micro-dots 5,they are reflected by the reflection slopes 11 to be thereby made toexit from the lower surface 10 of the light guide plate. Therefore, theabove-mentioned arrangement of the micro-dots is the most efficient.

Next, description will be made about the dimensions of the micro-dots 5.

Approximately, the short-side length W15 of each of the micro-dots 5 is,for example, in a range of from 0.002 to 0.05 mm, and the long-sidelength L16 thereof is, for example, in a range of from the short-sidelength to 0.2 mm, that is, in a range of from 0.002 to 0.2 mm.

This is because, for example, in the case where original shapes of themicro-dots are formed by use of a well-known photolithographic method,it is difficult to form each of the micro-dots with a desirable profileif the length of the micro-dot is not longer than 0.002 mm.

That is, the profiles of the micro-dots become irregular or the surfaceaccuracy of the dots in section is lowered due to the lowering of theresolution of a photo mask, the resolution of exposure or development,etc. As a result, the scattering of the light propagated inside thelight guide plate becomes so great that it becomes difficult to obtain alight guide plate which is high in light utilization efficiency.

On the other hand, the reason why an upper limit of the short-sidelength W15 of each of the micro-dots 5 is set to 0.05 mm is that nakedeyes have a visual limit of about 0.05 mm.

That is, if the short-side length of each of the micro-dots is notsmaller than 0.05 mm, the micro-dots themselves can be recognized, forexample, even with naked eyes (there occurs a phenomenon that themicro-dots 5 can be seen with naked eyes, that is, a phenomenon that thelight guide plate looks like an aggregate of point light sources) sothat the visibility as the liquid-crystal display device using themicro-dots is deteriorated.

On the other hand, the reason why the long-side length of each of themicro-dots 5 is set to be not shorter than the short-side length thereofis to increase the area of the reflection slopes 11 of the micro-dots.Thus, propagated light can be reflected effectively toward theliquid-crystal display device without increasing the number ofmicro-dots 5 to be formed on the surface of the light guide plate 1.

In addition, the reason why the long-side length of each of themicro-dots 5 is set to be not longer than 0.2 mm is as follows. If thelong-side length of each micro-dot 5 is longer than 0.2 mm, themicro-dots 5 themselves can be recognized with naked eyes to therebyspoil the visibility as the liquid-crystal display device. Further, inthe case where density distribution is applied to the way of arrangementof the micro-dots 5, it is difficult to give a gradient to the densitydistribution if the long-side length of each of the micro-dots 5 is notshorter than 0.2 mm. As a result, uniform distribution of illuminationlight cannot be obtained.

Next, description will be made about the depth H14 of the micro-dot 5.

The depth H14 of each of the micro-dots 5 is defined automatically basedon the dot short-side length W15 and the sectional inclination angle 12.It is, however, necessary to select the dot short-side length W15 andthe sectional inclination angle 12 so that the depth H14 is in a rangeof from 0.002 to 0.04 mm. That is, when the dot depth H14 is not largerthan 0.002 mm, the area of the reflection slope 11 of the dot 5 becomesso small that the function of changing the travelling direction of thelight incident on the light guide plate 1 is lost to reduce theutilization efficiency of light with which the liquid-crystal displaydevice 8 is irradiated.

On the other hand, when the depth W14 of the dot 5 is set to be notsmaller than 0.04 mm, the quantity of irradiation from the light guideplate 1 increases in an area near the light source 6, so that uniformirradiation becomes difficult.

Next, description will be made about the arrangement of the micro-dots5.

In conclusion, it is desirable that the micro-dots 5 are arranged withno regularity. This is because the micro-dots 5 described in thisembodiment are extremely minute so that the irregular arrangement of themicro-dots 5 is necessary for prevention of a moire phenomenon fromoccurring due to the interference of the micro-dots 5 with a regularlyformed pattern of a member constituting the liquid-crystal displaydevice 8, for example, represented by a liquid-crystal cell, a colorfilter, a TFT pattern, a black matrix, etc.

Particularly, when the illuminator is disposed in front of theliquid-crystal display device 8 in use, there is no diffusing plate,which would be often used on normal occasions, between the light guideplate 1 and the observer. Therefore, it is an extremely importantproblem to prevent such a moire phenomenon.

However, when the micro-dots 5 are arranged simply with irregularity, itis easy to produce a cluster 21 of micro-dots 5 or an area 22 wherethere is no micro-dot 5, as illustrated in FIG. 13. As a result, thevisibility as the liquid-crystal display device may be deteriorated.

It is therefore preferable that a radial distribution function similarto the case described in JP-A-10-1537519 is used to form the micro-dotsso as to satisfy the following conditions.

That is, in the light guide plate 1, the surface on which the micro-dots5 are formed is sectioned into 0.25 to 1 mm² square areas over an areanot smaller than 95% of the aforementioned surface on which themicro-dots 5 are formed. In each of the square areas, the dots 5 areformed and disposed so that a function G(R) which is obtained by takinga weighted average of a radial distribution function g(R) obtained foreach dot in accordance with the arrangement relationship of the dots,and which is obtained by approximating the weighted average by a leastsquares method satisfies the relation of 0<S₁/S₂<0.2 in a range ofR/R₀=3 to 6;

provided that R designates a distance from a central position of one dotto a central position of another dot; R₀, a value obtained by dividing alength of one side of the square area by a square root of the number ofthe dots existing in the square area; S₁, a value obtained byintegrating a difference between G(R) and an average value of G(R) withR/R₀ which is in a range of from 3 to 6; and S₂, a value obtained byintegrating the average value of G(R) with R/R₀ which is in a range offrom 3 to 6.

The reason why the aforementioned arrangement is required in an area notsmaller than 95% of the surface on which the micro-dots 5 are formed isthat a measure to prevent moire is required in the aforementioned areabecause the dots themselves may be observed directly when theilluminator is disposed in front of the liquid-crystal display device.Thus, the visibility as the liquid-crystal display device can beensured.

The area of each square is determined to include, preferably, at least10 micro-dots 5, more preferably at least 50 micro-dots 5 in the squarearea. That is, if the area of the square is not larger than 0.25 mm²,the number of dots included in the square area is too small to calculatethe radial distribution function g(R) because the value of R₀ is usuallyapproximately in a range of from 0.01 to 0.2 mm.

On the contrary, if the area of the square is set to be not smaller than1 mm², the quantity of the light radiated from the light guide plate 1cannot be estimated correctly when the dot density distribution ischanged to correct the light quantity. Thus, it may be difficult tocorrect the light quantity.

TABLE 1 Relationship between S1/S2 Value and Moire Occurrence itemAbsolute random-number arrangement method overlap no no 0.02 mm 0.02 mm0.03 mm 0.04 mm constraint S1/S2 0.8 0.5 0.5 0.3 0.2 0 moire ⊚ ⊚ ⊚ ⊚ ⊚ ⊚dot overlap × × × Δ ◯ ⊚ dark/bright × × ◯ ◯ ◯ ⊚ Spot total × × × Δ ◯ ⊚valuation dot dimensions 0.01 × 0.08 mm

Table 1 shows the results of investigation about the relationshipbetween the production of moire and the aforementioned coefficients,while the range of the value S₁/S₂ was determined on the basis of theresult described above. Further, the function G(R) is set to besubstantially 0 in the range of R<(short-side length of dot)×2. Thissetting is to prevent dot overlap, which is produced when dots are closeto each other, from being observed.

Incidentally, the overlap constraint in Table 1 is a method for defininga shortest distance between dots adjacent to each other so as to makethe function G(R) substantially 0 in the range of R<(short-side lengthof dot)×2.

Since the dot short-side length is 0.01 mm in this embodiment, thefunction G(R) can be made substantially 0 in the range of R<(short-sidelength of dot)×2 if the shortest distance between dots adjacent to eachother is set to be not shorter than 0.02 mm. The “spot” is that which isobtained by judgement as to whether a dark area where dots overlap witheach other or a bright area where there is no dot is generated and canbe visually confirmed or not.

Further, FIG. 14 shows the relationship between the function G(R) andthe distance R. As is apparent from this result, the aforementioned dotsare arranged so that at least two peaks each of which is at least twiceas large as the average value of the function G(R) exist in the functionG(R) in a range of R/R₀=3 to 6. The reason for such dot arrangement isthat, in the case where substantially rectangular dots are used, the dotdensity is easily enhanced if the dot interval on the short side is madeshorter than the dot interval on the long side.

On the other hand, the reason why each of the peaks is made at leasttwice as large as the average value of the function G(R) in a range ofR/R₀=3 to 6 is that, by adding such a condition, the distance (positionrelationship) between dots adjacent to each other can be keptsubstantially constant so that the production of a cluster of dots orthe production of an area where there is no dot can be prevented.

Next, description will be made about the method for manufacturing thelight guide plate according to this embodiment.

Fundamentally, a mold is first manufactured and, the light guide plateis then manufactured by performing plastic molding with the mold. Atthis time, as the method for manufacturing the mold, various well-knownmachining methods, such as drilling, cutting, grinding, etc., may beused. Alternatively, an electrical discharge machining method is alsoeffective.

The number of micro-dots is in a range of from 200 to 20,000 pieces/cm²in this embodiment, and hence it exceeds 1,000,000 pieces in total onthe whole surface of the light guide plate. Therefore, it should be,however, regarded as very difficult to form such a large number ofmicro-dots by the aforementioned manufacturing method.

FIGS. 15A to 15G show a process of forming a mask pattern on a siliconsubstrate, and FIGS. 15H-15M show a process of forming micro-dots by useof an anisotropic etching method.

This manufacturing method has the steps of:

(A) cutting a silicon substrate 30 out of a silicon single crystal ingot29 so that the silicon substrate 30 has a predetermined crystal plane;

(B) forming a silicon oxide film 31 on the surface of the siliconsubstrate 30 by use of a well-known method;

(C) forming a photo-resist film 32 on the silicon oxide film 31;

(D) disposing a photo mask 33 having a micro-dot pattern on the siliconsubstrate 30, and irradiating the resist film 32 with ultraviolet rays(UV) from above the mask 33 to thereby expose the resist film 32;

(E) developing the resist film 32, and forming a pattern 34 ofmicro-dots on the silicon oxide film 31;

(F) pasting a protective tape 35 on the silicon oxide film 311 which isformed on the back surface of the silicon substrate 30, and removing thesilicon oxide film 31 from portions of the silicon substrate 30 otherthan the back surface by a well-known etching method;

(G) removing the resist film 32;

(H) anisotropically etching the silicon substrate 30 with the pattern 34of the silicon oxide film 31 serving as a mask;

(I) removing the protective tape 35 and the silicon oxide film 31;

(J) forming a plating undercoat film 36 on the etched surface of thesilicon substrate 30 by use of a well-known method;

(K) forming a plating film 37 by use of a well-known plating method withthe plating undercoat film 36 serving as an electrode;

(L) stripping the plating film 37 off, and producing a stamper 38 havingthe micro-dot pattern 34;

then, performing abrasion on the micro-dot surface of the stamper 38 andthe back surface thereof in accordance with necessity (not-shown); and

(M) installing the stamper 38 in a well-known molding machine, andforming a light guide plate 1 by an injection molding method.

The respective steps will be described below in detail.

First, the step of cutting the silicon substrate 30 out of the siliconsingle crystal ingot 29, as shown in the step (A), is one of the mostimportant steps in the manufacturing method.

When micro-dots each having a substantially V-shape in section areformed on the surface of the light guide plate 1, differences in etchingspeed in accordance with crystal orientations of silicon crystals asshown in Table 2 are utilized. That is, even if etching is performed ona crystal with any crystal plane as the silicon substrate 30, a (111)plane for which the etching speed is lowest is finally set to havereflection slopes of the micro-dots formed.

TABLE 2 Etching Speed of Anisotropic Etching Etching speed Crystal plane(m/sec) 100 1.05E−05 110 2.15E−05 210 2.06E−05 211 1.64E−05 221 9.77E−06310 1.80E−05 311 1.78E−05 320 2.14E−05 331 1.41E−05 530 2.12E−05 5402.14E−05 111 1.50E−07 KOH 20 wt % 80°

By utilization of such a crystal characteristic, it is possible to formmicro-dots each of which has an optional sectional inclination angle andhas a substantially V-shape in section and the plan shape of which issubstantially rectangular.

FIG. 16A is a view showing a method for cutting out a crystal when thesectional inclination angle is 40° In this embodiment. FIG. 16B is aschematic view of the silicon substrate 30 cut out. FIG. 17 is a planview of the silicon substrate 30 cut out.

By performing anisotropic etching on this silicon substrate 30, it ispossible to form micro-dots having a predetermined sectional inclinationangle shown in FIG. 16B by way of example. Here, by changing the anglewith which the silicon substrate 30 is cut out, the silicon substrate 30can be manufactured to have an optional crystal plane. As a result, asdescribed above, it becomes possible to manufacture the siliconsubstrate 30 having a desirable sectional inclination angle inaccordance with the kind of light source. In addition, the crystal planeof the silicon single crystal is not limited to the (001) plane. Asilicon single crystal having an optional crystal plane may be used.Further, a silicon single crystal with a desired crystal planedetermined at the beginning may be manufactured and used.

For the step of forming the silicon oxide film 31 which is used as amask in anisotropic etching on the surface of the silicon substrate 30,as shown in the step (B), various methods may be used. In thisembodiment, al well-known thermal oxidation method was used. Table 3shows an example of thermal oxidation conditions.

TABLE 3 Example of Anisotropic Etching Process Step Conditions  1Forming thermally- temperature: 1,000° C., water oxidized film on Sitemperature: 90° C. wafer film thickness: 0.0005 mm, 31 min  2 Bakingbefore resist nitrogen atmosphere, 140° C. 30 min coating  3Photo-resist coating OFPR-8600 10 cp 1,000 rpm film thickness: 0.001 mm 4 Pre-baking nitrogen atmosphere, 90° C. 30 min  5 Exposure/developmentexposure: 50 mJ developer: NMD-3  6 Post-baking nitrogen atmosphere,140° C. 30 min  7 Oxygen plasma ashing 800W 400 sccm 3 min 45 s  8 Oxidefilm etching dip HF: NH₄F = 1:7 etching time: 6 min  9 pastingprotective film made by Nitto Electric film on back surface IndustrialCo., Ltd. 10 removing photo-resist S502A stripping liquid 110° C. 10 min11 Si anisotropic etching 20 wt % KOH 1.5 hr 12 removing oxide film dipHF: NH₄F = 1:7 etching time: 6 min 13 washing/drying vapor washing: 5min 14 forming thermally- Temperature: 1,000° C., water oxidized filmagain temperature 90° C. film thickness: 0.001 mm, 60 min

Next, in the step (C) of forming the photoresist film 32 on the siliconoxide film 31, it is preferable that a primer is applied to the siliconoxide film 31 as a pre-process so as to improve the adhesion to anundercoat film. As a proper method for the primer treatment, variousmethods may be used. For example, when a silane agent is used as theprimer, hexamethylsilazane is suitable. That is, a so-called gaseousdiffusion process is used so that the hexamethylsilazane is supplied toa vessel and evaporated to form a thin film on the substrate surface.Thus, a uniform film can be formed on the silicon oxide film 31.

As the photo-resist material, for example, a fluid-like or film-likepositive type or negative type material may be used. In FIG. 15C, apositive type material was formed by use of a well-known spin-coatingmethod.

In the step (D), for example, a chromium mask, a film mask, an emulsionmask, etc. may be used as the photo mask. Data such as the size andnumber of designed micro-dots, the distribution thereof, and so on, areprepared in advance. A pattern of the micro-dots is drawn, for example,by use of an electron beam method, a laser beam method, or the like.This pattern is used as the mask.

In the steps (E) to (G), exposure, etching of the silicon oxide film 31,and removal of the resist film 32 are performed by well-known methodsrespectively.

By the above steps, the silicon substrate 30 having a predeterminedmicro-dot pattern in the silicon oxide film 31 is completed.

Next, as shown in the step (H), anisotropic etching is performed on thesilicon substrate 30 with the pattern of the silicon oxide film 31serving as a mask. Table 3 shows an example of etching processconditions. A KOH solution the KOH concentration of which was about 20%was used as etching liquid. In such conditions, micro-dots having aV-shape in section with a sectional inclination angle of about 40° wereformed on the surface of the silicon substrate 30.

Succeedingly, in the step (I), the protective tape 35 formed on the backsurface of the silicon substrate 30, and the silicon oxide film 31 areremoved by use of a well-known method. Then, a plating layer (stamper)is formed by a plating method shown in the steps (J) and (K).Incidentally, if the undercoat film 36 is formed on the siliconsubstrate 30 having the micro-dots 30 in advance, unevenness of theplating film can be reduced in the plating step so that a superiorplating layer, that is, a superior stamper can be formed.

Although the aforementioned undercoat film may be formed by use of awell-known plating method or may be formed of a sputter film such as anNi thin-film or the like, this film thickness is an extremely importantparameter. That is, if the film thickness is large, there arises aproblem that the thin film is stripped off in the plating step.

Therefore, in this embodiment, the film thickness was controlled to bein a range of from 0.015 to 0.035 μm, especially in a range of from 0.02to 0.03 μm. If the film thickness is out of this range, there arises aproblem that uniform plating processing becomes impossible (if theundercoat film thickness is thin), or the undercoat film 36 or theplating film 37 which is formed with a micro-dot pattern is stripped off(if the undercoat film thickness is thick).

Although various metals may be used as the material for the undercoatfilm 36 and the plating layer 37 formed by the plating method, Nimaterial was used here in consideration of uniformity of film thicknessand mechanical performance.

Next, as shown in the step (L), the obtained plating film 37 is strippedoff from the silicon substrate 30 so as to be used as the stamper 38 forforming micro-dots in the surface of the light guide plate. At thistime, in order to obtain a light guide plate with a high lightutilization efficiency, it is important to perform abrasion on themicro-dot surface. Therefore, abrasion was performed with aluminaabrasive grains the average grain size of which was in a range of from0.1 to 1 μm in this embodiment. However, it is not limited to the aboveabrasion, hand lapping or machine lapping with diamond abrasive grainsmay be performed.

Finally, as shown in the step (M), for example, the obtained stamper isfixed to a matrix of an injection molder by a magnet, a vacuum chuck, orthe like, and a material to form the light guide plate is supplied tothe matrix. Thus, the light guide plate having micro-dots withpredetermined dimensions is completed. Incidentally, extrusion molding,compression molding, vacuum molding, or the like, which are known well,may be used as the molding method.

General transparent plastic materials are available as the material toform the light guide plate. Specific examples of available materialsinclude acrylic plastic, polycarbonate resin, polyacetal resin,polyolefin resin, ultraviolet-curing plastic material. Particularly,since acrylic resin material is superior in transparency, price,moldability, and so on, it is a material suitable for manufacturing thelight guide plate according to this embodiment.

Next, description will be made about a second embodiment where theaforementioned light guide plate has been applied to a liquid-crystaldisplay device, with reference to FIG. 18.

FIG. 18 is a schematic sectional view of a liquid-crystal displaydevice. On the lower surface of the light guide plate 1, there aredisposed a polarizer 40, a phase-difference film 41, a diffusing film42, a glass substrate 43, a color filter 44, a pixel electrode 45, aliquid-crystal cell 46, a TFT 47, a reflection polarizer 48, anabsorbing film 48, etc. This configuration shows an example of awell-known reflection type liquid-crystal display device. Variousconfigurations may be considered in accordance with applications ofliquid-crystal display devices.

Specific examples of a light source 6 include a cathode-ray tube, alight emission diode, an EL element, etc. as mentioned above. A suitablelight source is selected from the point of view of power consumption,use form, etc. In this embodiment, five light emission diodes were used.In addition, optical parts including the liquid-crystal cell 46 are notlimited specifically. Well-known parts were used for the optical parts.

The light guide plate measured about 30×30×1 mm. Each of micro-dots 5which is formed on the surface of the light guide plate 1 measured 0.01mm in dot short-side length, 0.08 mm in dot long-side length, 40° insectional inclination angle, and 70.6° in vertex angle. Particularly,the sectional inclination angle was set to be 40° so as to make thelight divergent angle in a range of about ±25° and so as to restrain thedivergence of the exiting angle from the light guide plate 1 to be smallbecause the light sources 6 were made of light emission diodes (seeFIGS. 8A to 8C and FIGS. 9A and 9B).

FIG. 19 is a view conceptually showing a section of a liquid-crystaldisplay device in front of which the aforementioned illuminator has beendisposed. Other than the illuminator 23 disposed in front of theliquid-crystal display device 8, there are provided a driving circuit 26for driving the liquid-crystal display device 8, a control circuit 24and a power supply 25 for driving the liquid-crystal display device 8and the driving circuit 26.

Under a normal usage environment, for example, when the liquid-crystaldisplay device 8 is used indoors or outdoors with sufficient externallight, display showing sufficient luminance can be performed withoutusing the illuminator 23. If the light quantity from the external lightis insufficient, the illuminator 23 is controlled by the control circuit24 so that required light can be supplied from the light sources to theliquid-crystal display device 8.

When the liquid-crystal display device was irradiated with the lightfrom the light sources in addition to the external light with the aboveconfiguration, the liquid-crystal display device exerted much highervisibility than a background-art liquid-crystal display device having nolight guide plate. Thus, high luminance display could be attained. Sinceit will go well if the illuminator 23 is operated in accordance withnecessity, the power consumption can be reduced while visibilityrequired of the liquid-crystal display device is ensured.

FIG. 20 is a conceptual view for explaining a third embodiment.

In this embodiment, a portable electronic apparatus in which theilluminator 23 has been disposed in front of the liquid-crystal displaydevice 8 is illustrated. In addition to the configuration of theembodiment shown in FIG. 20, the portable electronic apparatus has alight receiving element 27 disposed on the side facing theliquid-crystal display device 8. Specifically, a mobile liquid-crystaldisplay device, a portable telephone, or the like, is assumed.

In this case, for example, external light, for example, sunlight or thelike, is received also in the light receiving element 27, and anelectric signal converted in the light receiving element 27 is suppliedto the control circuit 24. Then, the control circuit 24 controls theilluminator 23 in accordance with the magnitude of this electric signalso as to adjust the quantity of light with which the liquid-crystaldisplay device 8 is irradiated.

FIG. 21 is a view conceptually showing the relationship between thequantity of external light and the luminance of the liquid-crystaldisplay device. The abscissa designates that the quantity of externallight increases as the abscissa goes toward the left side. As isapparent from this drawing, when the quantity of external light islarge, the frontal luminance of the liquid-crystal display device can beensured sufficiently so that the illuminator does not have to beoperated.

However, as the quantity of external light decreases, the frontalluminance of the liquid-crystal display device is lowered. In thisstate, sufficient frontal luminance of the liquid-crystal display devicecannot be ensured so that the visibility of display is remarkablylowered.

In such a case, when the illuminator is operated in accordance with theoperation curve of the illuminator as illustrated in FIG. 21, sufficientvisibility of display can be attained even if the quantity of externallight is insufficient. In addition, the liquid-crystal display devicecan be controlled to make display with substantially constant luminanceregardless of the quantity of external light.

As has been described above, according to this embodiment, light can beautomatically supplied from the illuminator in accordance with thequantity of external light. Accordingly, the visibility of theliquid-crystal display device can be enhanced while the convenience ofbeing a portable apparatus is ensured.

Incidentally, the aforementioned embodiment is only an example, and notto say, the present invention is not limited to the embodiment. Forexample, a receiving terminal or a receiving device for receiving aninformation signal may be further provided so that, when an informationsignal is received, the control circuit controls the illuminator(including the light receiving element) with this signal as a trigger tothereby adjust the frontal luminance of the liquid-crystal displaydevice. Alternatively, an observer may use a switch or a volume to beable to adjust the frontal luminance of the liquid-crystal displaydevice as the occasion demands.

As has been described above, the luminance of a display screen can beenhanced by disposing an illuminator according to the present inventionin front of a liquid-crystal display device. In addition, a light guideplate which is high in light utilization efficiency can be manufacturedby using an anisotropic etching method on a silicon substrate having apredetermined crystal plane.

While we have shown and described several embodiments in accordance withour invention, it should be understood that disclosed embodiments aresusceptible of changes and modifications without departing from thescope of the invention. Therefore, we do not intend to be bound by thedetails shown and described herein but intend to cover all such changesand modifications a fall within the ambit of the appended claims.

We claim:
 1. A front-lighting illuminator, comprising: a front lightguide plate; and a light source disposed on one of side surfaces of saidfront light guide plate; wherein said front light guide plate includesan incidence surface on which light from said light source is incident,and two light transmission surfaces, said incident light being made toexit from one of said two transmission surfaces thereof and to enter aliquid-crystal cell through a front surface thereof which is disposedbehind said front light guide plate; and wherein a plurality of dotseach constituted by a small recess or protrusion portion for reflectingsaid light from said incidence surface toward said one lighttransmission surface are formed in or on another surface of said twolight transmission surfaces of said front light guide plate opposite tosaid one light transmission surface, each of said dots beingsubstantially rectangular in plan view and being substantially V-shapein section with a vertex angle of about 70.6°±2.5°, and an inclinationangle of said section being in a range of from 35°to 43°so as to enablean increase of light quantity exiting from said front light guide plateand entering said liquid crystal cell.
 2. A front-lighting illuminatordisposed on a front surface of a liquid-crystal cell, comprising: afront light guide plate; and a light source disposed on one of sidesurfaces of said front light guide plate; wherein said front light guideplate includes an incidence surface on which light from said lightsource is incident, and two light transmission surfaces, said incidentlight being made to exit from one of said two transmission surfaces ofsaid front light guide plate and to enter said liquid-crystal cellthrough the front surface thereof; and wherein a plurality of dots eachconstituted by a small recess or protrusion portion for reflecting saidlight from said incidence surface toward said one light transmissionsurface are formed in or on another surface of said two transmissionsurfaces of said light guide plate opposite to said one lighttransmission surface, each of said dots being substantially rectangularin plan view and being substantially V-shape in section with a vertexangle of about 70.6°±2.5°, an inclination angle of said section being ina range of from 35°to 43°, an area not smaller than 95% of a whole areaof the surface where said dots are formed is sectioned into 0.25 to 1mm² square areas, and said dots are disposed in each of said squareareas so that a function G(R) which is obtained by taking a weightedaverage of a radial distribution function g(R) obtained for each of saiddots in accordance with an arrangement relationship of said dots andwhich is obtained by approximating said weighted average by a leastsquares method satisfies a relation of 0<S₁/S₂<0.2 in a range of R/R₀ =3to 6 so as to enable an increase of light quantity exiting from saidfront light guide plate entering said liquid-crystal cell; provided thatR designates a distance from a central position of one dot to a centralposition of another dot; R₀, a value obtained by dividing a length ofone side of said square area by a square root of the number of said dotsexisting in said square area; S₁, a value obtained by integrating adifference between G(R) and an average value of G(R) with R/R0 which isin a range of from 3 to 6; and S₂, a value obtained by integrating saidaverage value of G(R) with R/R₀ which is in a range of from 3 to
 6. 3. Afront-lighting illuminator according to claim 1, wherein: each of saiddots has a short-side length in a range of from 0.002 to 0.05 mm and along-side length in a range of from 0.002 to 0.2 mm.
 4. A front-lightingilluminator according to claim 1, wherein: said dots are disposed atrandom in or on the other surface of said light guide plate opposite tosaid light transmission surface.
 5. A front-lighting illuminatoraccording to claim 2, wherein: each of said dots is disposed so thatsaid function G(R) is substantially 0 in a range of R<(short-side lengthof said dot)X2, at least two peaks exist in said function G(R), and saidtwo peaks each of which said two peaks is at least twice as large assaid average value of said function G(R) exist in a range of R/R₀ =3 to6.